This document discusses the use of surface electromyography (EMG) in biomechanics. It reviews many factors that can influence the EMG signal and force produced by a muscle, including intrinsic physiological factors like motor unit firing characteristics and extrinsic technical factors like electrode placement. The document focuses on three main applications of surface EMG: determining muscle activation timing, relating EMG to muscle force, and using EMG as a fatigue index. It provides recommendations for proper use and identifies outstanding challenges in the field.